Temperature measurement is important in many fields. This is especially true in the medical field where heat is often utilized, including for example, for sterilization of equipment and for patient therapy. Many physicians and physical therapists prefer the use of moist heat to other forms of heat for patient therapy, and to effect this end, hotpacks have herefore been developed and utilized in medical treatment such as, for example, in the treatment of arthritis and for muscle therapy.
Hotpacks conventionally include a cover having one or more pockets, or cells, contained therein, each of which contain a heat retaining material. The hotpacks are placed in a heated liquid, such as hot water, until the heat retaining material within the cells absorb sufficient heated water to raise the temperature of the hotpack to a desired level. One widely used heat retaining material is clay, such as bentonite, since bentonite, in addition to retaining heat from the absorbed liquid, also expands upon absorption of the liquid to fill the cells in which it is enclosed.
A major problem in the use of hotpacks has been the problem of determining when the heat retaining material in the hotpack has reached a temperature level sufficient to be effectively utilized to impart moist heat to a patient. In the past, this determination has largely been made on a subjective basis. That is, the hotpacks were heated for a time period which was thought to raise the temperature of the heat retaining material to the desired level. Thus, due to environmental and other factors, a user could not be certain that the hotpack was even close to the desired temperature. Often a number of hotpacks were treated in a single container of liquid. Unless the hotpacks were all placed into the container at the same time, however, it was difficult, if not impossible, to distinguish how long a particular hotpack had been exposed to the heating process. It was therefore exceedingly difficult with prior art hotpacks to insure safe and effective use at least without utilizing other cumbersome and/or elaborate systems.
Devices are known which provide an indication of a predetermined temperature having been reached. One such device is shown in U.S. Pat. No. 2,826,073. That device is a sterilization indicator which includes strips of paper or similar materials which are placed in packs of medical instruments. A thermosensitive indicating material on the indicator inserted in the packs indicates that the materials to be sterilized have been exposed to steam for a period of time sufficient to attain sterilization. Thus, this device measures the temperature of the steam or liquid which is used to sterilize instruments, but does not measure the internal temperature of the instruments.
A device for determining the internal temperature of an object is disclosed in U.S. Pat. No. 4,137,769. That device employs a model of the object for which it is desired that the internal temperature be known. The model is placed in the heating medium along with the object. The model includes a thermochromic material known in the prior art which changes color at different temperatures. The thermochromic material may be in the shape of numbers to change colors and become visible at a temperature indicated by a particular number, but measurement of the internal temperature of objects is restricted to measurement only of the internal temperature of the model and then is only valid if the objects and model are subjected to the same heat for the same period of time.
Other known devices, such as heat thermometers, which might be thrust into the center of a material whose temperature is to be measured, are impractical for use with hotpacks in that they would destroy the integrity of the cover which must remain sufficiently intact to retain the heat retaining material (but yet must be liquid permeable to allow liquid to pass into and out of the enclosure having the heat retaining material therein).